Proposed “The George” Mixed Use Development 377, 385 & 407 Gower Point Road, 397 & 689 Winn Road, and Winn Road Right-of-Way, Gibsons, BC SUMMARY OF GEOTECHNICAL INVESTIGATION REPORT (REVISED) ISSUED APRIL 7, 2015 Karim Karimzadegan, M.A.Sc., P.Eng. Jean Cho, Ph.D, P.Eng. Horizon Engineering Inc . May 12, 2015
BACKGROUND DISCUSSION • General site description
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information THE GEORGE SITE
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information THE GEORGE SITE
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information • Site investigations – September 2012 (test pits and auger test holes) – April 2014 (auger test holes and sonic boreholes) – December 2014 (sonic boreholes) – January 2015 (WildCat holes)
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information • Site investigations • Previous geotechnical investigation reports – Horizon (2012, 2014) – Thurber (2007) – Geotactics (2004) • Proposed development – A new multi-level hotel, – Conference centre – Residential development (partially over below grade parkade) – At-grade café, retail space, and meeting room – Over-water restaurant building and marina
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information • Site investigations • Previous geotechnical investigation reports • Proposed development • Soil conditions – Surficial Soil (topsoil / fill) – Sand/Silty Sand/Peat/Till (Aquitard) – Sand/Gravelly Sand (Aquifer) • Groundwater conditions – Non-artesian – Artesian • Groundwater monitoring
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information • Site investigations • Previous geotechnical investigation reports • Proposed development • Soil conditions • Groundwater conditions • Groundwater monitoring
BACKGROUND DISCUSSION • General site description • Site geology and hydrogeology • Background information • Site investigations • Previous geotechnical investigation reports • Proposed development • Soil conditions • Groundwater conditions • Groundwater monitoring
HYDROGEOLOGY DISCUSSION Gibsons Groundwater Resource Source: Doyle (2013) UBC Master’s Thesis
THE GEORGE SITE Gibsons aquifer recharge is approximately 45% through Elphinstone mountain block and 55% through Upper Gibsons bench. Project Area is in the discharge area. Source: Doyle (2013) UBC Master’s Thesis
Numerical Model of Gibsons Groundwater Resource indicates that the project area is in a groundwater discharge zone (i.e., zero recharge). Source: Doyle (2013) UBC Master’s Thesis and Gibsons Aquifer Study (2013)
Numerical Model of Gibsons Groundwater Resource indicates that the well-head protection zone does not include the project area. Source: Doyle (2013) UBC Master’s Thesis
BH14-5, 90 m from the harbour, shows a direct connection with the tidal signature in Howe Sound.
BH14-6, 120 m from the harbour, shows a direct but more muted connection to the tidal signature in Howe Sound than BH14-5.
BH14-4 shows a muted tidal signature and strong apparent correlation with pumping at Town Well #1.
POTENTIAL GEOTECHNICAL CHALLENGES • Artesian groundwater pressures • High non-artesian groundwater levels • Sea level rise • Tsunami hazard • Loose and compressible soil • Liquefaction • Impact of dredging on foreshore area • Methane buildup
ARTESIAN GROUNDWATER PRESSURES • Artesian groundwater pressures have been observed within the Gibsons Aquifer. • Hydraulic connections have been observed between the Gibsons Aquifer and the ocean at the central portion of the site and between the Gibsons Aquifer and Town Well #1 at the west portion of the site. • A computer model was generated to analyse the site and subsurface conditions during and after construction of the proposed development based on existing information, published literature, and engineering judgement. • The results of this modelling work indicate that the proposed excavation should not advance below a geodetic elevation of 5.0 metres (16 feet 5 inches) at the northwest portion of the site in order to ensure that the underlying Gibsons Aquifer is not compromised (even temporarily) due to excavation of the overlying materials. • The results of these analyses are based on conservative soil strength properties. Therefore, there is an inherent Factor of Safety (which may be of the order of approximately two) in the deformation analysis results.
ARTESIAN GROUNDWATER PRESSURES • At the southwest, southeast, and northeast portions of the site, we recommend that the proposed excavation not advance below 0.5 metre (1 foot 8 inches) below existing grades in order to ensure that the Gibsons Aquifer is not compromised due to excavation of the overlying materials. • Deeper excavation at the southwest portion of the site is not recommended due to the proximity of the Gibsons Aquifer to the existing site grades.
EXCAVATION
SEA LEVEL RISE AND TSUNAMI HAZARD • All habitable spaces are recommended to be constructed at or above a Flood Construction Level (FCL) of approximately 5.33 metres (17 feet 6 inches), which takes into account potential effects of sea level rise and storm and tsunami waves during the design life of the proposed building. • We envisage that habitable spaces could be constructed below the FCL if a sea dike is constructed around the building, which would be designed to protect the building from rising sea levels and future storm events. The proposed marina and over-water restaurant should be constructed at or above the FCL since they would otherwise be unprotected from the design flood conditions.
PROPOSED BUILDING ELEVATIONS • We envisage that the lowest proposed top of slab elevations would be approximately 6.3 metres (20.5 feet) at the west portion of the site and 3.1 metres (10.2 feet) at the east portion of the site. • Accordingly, we envisage that the proposed footing elevations would be approximately 5.4 metres (17.5 feet) at the west portion of the site and 2.2 metres (7.2 feet) at the east portion of the site. • Therefore, we envisage that the proposed excavation would be approximately 5.1 to 5.8 metres (17 to 19 feet) deep below adjacent existing grades at the northwest portion of the site. • Excavation at the southwest, northeast, and southeast portions of the site is envisaged to be less than approximately 0.5 metres (1.6 feet) deep below adjacent existing grades.
PROPOSED BUILDING ELEVATIONS • We envisage that the proposed finished floor elevation for the proposed café, retail space, meeting room, seawalk, and over-water restaurant at the east portion of the site would be approximately 5.3 metres (17.3 feet), which is consistent with the recommended FCL. The currently proposed lowest parkade floor elevation is below the FCL; therefore, a sea dike is envisaged to be required as part of the proposed development.
LOOSE AND COMPRESSIBLE SOILS • We recommend that foundations for the entire building footprint are supported on conventional strip and pad foundations or on a raft foundation. • Due to the presence of loose and compressible subgrade materials (which are judged to be unsuitable for supporting shallow foundations in their current state), ground improvement is recommended beneath proposed foundations such that suitable bearing is achieved. • We envisage that deep soil mixing may be the preferred method of ground improvement at the subject site.
LOOSE AND COMPRESSIBLE SOILS • We recommend that foundations for the entire building footprint are supported on conventional strip and pad foundations or on a raft foundation. • Due to the presence of loose and compressible subgrade materials (which are judged to be unsuitable for supporting shallow foundations in their current state), ground improvement is recommended beneath proposed foundations such that suitable bearing is achieved. • We envisage that deep soil mixing may be the preferred method of ground improvement at the subject site. • We recommend that footings proposed at the east portion of the site, where excavation is not required, are lowered to the existing grades after the proposed ground improvement measures are complete. In these areas, floor slabs are recommended to be designed as suspended slabs.
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